In preparing polymers (e.g. in polycondensation reactions) some of the process steps, such as the evaporation of solvents and of monomers, are applied to viscous media.
For these process steps, which are a function of mass transfer between a liquid phase and a gas phase, the product of the interfacial area between these two phases and the mass transfer coefficient is rate-determining.
In the case of mass transfer in the viscous liquids, the mass transfer coefficient is essentially controlled by the frequency of renewal of the surfaces of the liquid.
Renewal of surfaces may in principle take place by two different mechanisms:
by spreading out the liquid at a fixed surface (e.g. described in U.S. Pat. No. 5,399,012) or PA1 by allowing liquids to flow as free liquid films (as mentioned in U.S. Pat. Nos. 5,779,986 or 5,599,507). PA1 the distance between the mixer elements, PA1 free cross section of the mixer elements, PA1 the diameter of the openings in the mixer elements, PA1 the size of the openings in the overflow weirs.
The creation and renewal of the liquid surfaces by producing free films here has the advantage firstly that apparatus cost remains low and secondly that little mechanical energy has to be introduced into the materials of the mix.
Good kinematic self-cleaning of the internals of the mixing apparatus is also necessary in order to avoid the formation of deposits on the internals in which, if the mixer is used as a reactor, the long residence time of the reactants favours undesirable side-reactions.
In the German patent application of file number 19751730.7 an apparatus is disclosed which is both essentially kinematically self-cleaning and, as a result of film formation, also continuously produces renewed liquid surfaces for mass transfer to and from a gas phase.
Both in degassing procedures and in polymerizations there is frequently a rise in viscosity within a mixer by a factor of more than 10. A mixer which utilizes free-falling films to create and renew liquid surfaces cannot operate ideally unless the shape of the mixer elements is varied along the length.
To this end, in U.S. Pat. No. 5,599,507 variations are made, along the length of the apparatus in some or all of:
In the German Patent Application of file number 19751730.7 the plate width and the sizes of the openings in the discs can be varied along the length, but not the number of openings per plate.